Add 1 tablespoon of oil to a frying pan over medium heat. When the oil is warm, add the onions. Cook, stirring occasionally, until the onions are brown but not burned, 4-5 minutes. Stir in the vinegar, and cook until the liquid evaporates. Transfer the mixture to a food processor fitted with the metal blade, along with the remaining tablespoon of oil, and puree. Scrape the puree into the slow cooker. Do not wash the food processor.

Now, add the ginger, garlic, chile peppers, turmeric, coriander, garam masala and cinnamon to the food processor. Process into a paste. Scrape the paste into the slow cooker.

With a mortar and pestle or a dedicated spice grinder (a coffee grinder works well, as long as you're not using it for coffee!), combine the black peppercorns, salt and mustard seed, and crush together into a coarse powder. Add that mixture to the slow cooker.

Add the chicken and 1/4 cup of water to the slow cooker and, with a rubber spatula or wooden spoon, stir everything together. Set the cooker to LOW and cook for 6 hours. Lift the lid (yes, it's fine to do that) after the first 4-1/2 hours, and give the vindaloo a good stir.